The samples analysed in this work were selected from a collection of 35 Fe–Mn crusts collected from six CISP seamounts. The Fe–Mn crusts were collected in 2011 using a rectangular rock dredge (2 m × 0.8 m) on board the R/V Miguel Oliver during DRAGO0511 cruise. Samples and swath bathymetric data are part of the new extensive data set acquired for the Law of the Sea extended continental shelf (ECS) purposes. Sampling targets were selected after a previous seabed mapping using the Konsberg-Simrad EM-302 multibeam echosounder [59
]. 3D images of the seamounts were taken using Fledermaus™ software (Quality Positioning Services BV (QPS), Zeist, The Netherlands).
Two samples were selected from The Paps (DR07-8) and Tropic (DR16-13) Seamounts, respectively (Figure 1
B,C) on the basis of previous studies of the Fe–Mn crusts [27
], which highlighted important geochemistry differences, i.e., Mn contents (15 and 21.7 wt %, respectively), Fe (14.6 and 27.3 wt %), and especially in Co (3500 and 7000 µg/g), Ni (6000 and 3000 µg/g), Cu (1600 and 400 µg/g) and Ce (700 and 2100 µg/g) [27
]. These differences indicate that both samples could represent endpoint members between mixed diagenetic and purely hydrogenetic Fe–Mn crusts.
The Paps Seamount displays in the swath bathymetry and 3D images (Figure 1
B) a N-S ridge-like morphology with a narrow arm extending in a NW-SE direction. This seamount rises from 4400 m depth to 3000 m at the northern summit and extends over an area of 2150 km2
. Fe–Mn crusts were dredged on the northern flank (dredge samples DR07, DR09, DR10 and DR11) and on the southern arm (DR14) (Figure 1
B). The sample selected for this study belongs to the DR07 dredge, collected in the northern sector of the seamount at 1860 m depth (Figure 1
B). Calculated 40
Ar age in several feldspar micro-phenocrysts of the Paps Seamount gave a mean age of 91 Ma [39
The base of the Tropic Seamount, found at 4300 m depth, occupies an area of 1530 km2
C). This seamount of 3300 m in height shows typical guyot morphology with a “mesa”-type flat summit surrounded by steep flanks. In plain view, it shows a four-arm star caused by the action of landslide scars [59
]. Two dredges were collected on this seamount: dredge DR15 on the southern arm at 2200 m depth and, dredge DR16 on the eastern arm at 1700 m depth. The sample selected for this study belongs to dredge DR16 (Figure 1
C). The mean age of this seamount has been estimated as 119 Ma at the top of the seamount and around 80 Ma on the northern arm [39
3.2. Laboratory Methods
Petrographic, mineralogical and geochemical methods were performed at the Central Laboratories of the Geological Survey of Spain (IGME), the Department of Crystallography and Mineralogy, Faculty of Geosciences (UCM) and the National Centre of Electronic Microscopy (CNME). Mineralogical identifications were performed by X-ray powder diffraction (XRD, Philips Analytical, Almelo, The Netherlands) based on 12 sub-samples. The equipment used included a PANalytical X’Pert PRO diffractometer, CuKα radiation, carbon monochromator and automatic slit (PTRX-004). The analytical conditions for the XRD were as follows: CuKα radiation at 40 kV and 30 mA, a curved graphite secondary monochromator, scans from 2–70° (2θ), step size of 0.0170° (2θ) and step time 0.5°/min. An analysis was conducted on a bulk sample at room temperature and also dried at 40, 100 and 300 °C. The cation exchange experiment (CEE) was used to verify the presence of phyllomanganates in crusts with interlayer regions accessible for cation exchange of K+
ions as described by [60
] and subsequently used by [17
Scanning electron Microscope (SEM) images and analysis were obtained with a JEOL JSM 6335F (JEOL, Tokyo, Japan) with a cold (cathode) field-emission electron gun performed on up to 8 fragments of crusts. The SEM max resolution was 1.5 nm with 15 kV voltage at 4 mm distance. The backscattered electron detector worked with 2 nm resolution at 8 mm distance. The energy-dispersive X-ray spectroscopy (EDS, Oxford Instruments, Abingdon, UK) qualitative analysis was conducted with an Oxford Instruments, model: X-Max 80 mm2 with a resolution of 127 eV to 5.9 keV and performed with INCA software (ETAS GmbH, Stuttgart, Germany). This study was based mostly on the 58 SEM photomicrographs that were taken from these samples.
The high-resolution transmission electron microscope (HR-TEM) used was a JEOL JEM 3000F (JEOL, Tokyo, Japan) with a Schottky-type field emission electron gun at an accelerating voltage of 300 kV, 0.17 nm resolution. A micro-analysis system was implemented by an XEDS detector (OXFORD INCA) and an ENFINE spectrometer (resolution in energy of 1.3 eV). The study was carried out in 10 powdered sub-samples.
An electron probe micro-analyser (EPMA) was applied on polished thin sections using a Jeol JXA-8900M Electron Probe WDS/EDS Micro Analyser (JEOL, Tokyo, Japan), operating at 15–20 kV and 50 mA, operating at 15 EDS, equipped with four wavelength dispersion spectrometers in which these crystals were placed, as follows: channel 1: TAP; channel 2: LIF; channel 3: PETJ; channel 4: PETH. Standards included pure metals, synthetic and natural minerals, all from international suppliers. Back-scattered electron images were also obtained with this instrument. Profiles across crust samples were obtained to check the presence of compositional zoning and layer-by-layer growth rate and age determinations. Chemical EPMA determinations were carried out in 100 spot analyses in mineral phases, 183 and 292 layer-by-layer analysis in crusts DR07-8 and DR16-13, respectively. One hundred photomicrographs of mineralogical and textural features were taken with this instrument.
The major elements for both bulk Fe–Mn crusts were determined using X-ray Fluorescence (XRF, Philips Analytical, Almelo, The Netherlands), PANalytical’s Magix equipment with a rhodium tube and Major software (PTE-RX-001 Ed. 3) and Irons protocol. The accuracy of the data was verified using international standard NOD-A-1, and precision based on duplicate samples was found to be better than ±5%. Analytical conditions were 50 kV voltage and 50 mA. Na was measured using a VARIAN FS-220 atomic absorption spectrometer and loss on ignition (LOI) was determined by calcination at 950 °C. ICP-AES (Varian Vista-MPX, Varian Inc., Palo Alto, CA, USA) was used to measure Nb, S and W in bulk samples. Samples were prepared with acid HF HNO3 and HClO4 digestion until completely dried and residual was diluted with ClH 10%. Other trace Elements Be, V, Co, Ni, Cu, Zn, As, Se, Mo, Ag, Cd, Sb, Ba, Tl, Pb, Th, U and REEs plus Y were measured with ICP-MS (AGILENT 7500 CE, Agilent Technologies, Santa Clara, CA, USA) with 3-acid and 4-acid digestion. The patterns obtained were compared with certified international standards (NOD-A-1, USGS, NBS, CANMET, BCS). The standard reference materials SO-1 (CCMET), GSP-1 (USGS) and BCR-1 (USGS) were used to test the analytical procedure for REY determinations.
Sequential leaching was adapted from [61
], and according to [44
] method for Fe–Mn crusts, essentially used for different acid digestions. Acetic acid at 10% concentration was used to extract carbonates, 1 M hydroxylamine hydrochloride was used to extract Mn oxides, concentrated HCl was used to extract Fe oxyhydroxides and finally concentrated HF was used to digest residual silicates. Every solution was analysed by ICP-AES for Al, Ca, Fe, K, Mg, Mn, Na, Nb, P, S and W; and by ICP-MS for Be, V, Co, Ni, Cu, Zn, As, Se, Mo, Ag, Cd, Sb, Ba, Tl, Pb, Th, U and REY.
Crust growth rates and ages were calculated by the empirical Co-chronometer method established by [62
] which closely match isotopic determinations. The growth rate is (mm/My) = 0.68/Con1.67
, where Con
is (Co wt % × 50)/(Fe + Mn wt %). One limitation of this Co-chronometer method is that the equation does not measure possible hiatuses during the accretion process. Therefore, the calculated rates represent maximum values and the derived ages minimum values; if dissolution and re-precipitation take place can accumulate higher growth rates relative to diagenesis [63
]. Multi-elemental spot analyses of Fe, Mn, and Co were carried out across two continuous profiles from the base to the edge of the crusts using electron probe microanalyzer (EPMA, JEOL, Tokyo, Japan) with a JEOL JXA-8900M Superprobe. Analysis is based on the study of 183 and 292 layer-by-layer analysis for the EPMA profiles of ferromanganese crusts DR07-8 and DR16-13, respectively.
The varimax rotated factor matrix was used to calculate different factors found in the study samples for Mn, Fe, Si, Al, K, Ca, Na, Mg, P, Co, Ni, Cu, Ce, Mo, V, Ba and W. DR07-8 factor was obtained with 183 layer-by-layer EPMA analyses while DR16-13 factor was obtained with 292 layer-by-layer analyses. More factors have been obtained by Varimax in both crusts but all of them have counts down to 5% and considered not representative by the statistical program. These non-representative factors have not been included in the factor list.